Asynchronous invocation

If used inside a single-threaded xp-forge/web web application, waiting will block the entire server. We should also have an asynchronous method to execute requests and wait for them.

Blocks server until API call returns

Current functionality

function($req, $res) use($endpoint) {
  $user= $endpoint->resource('/users/me')->get()->value();
  
  $res->answer(200);
  $res->write('Hello '.$user['displayName'], 'text/plain');
}

Asynchronous handler function

Idea: Defer reading from request until any of RestResponse's methods are called, add await():

function($req, $res) use($endpoint) {
  $user= (yield $endpoint->resource('/users/me')->get()->await())->value();
  
  $res->answer(200);
  $res->write('Hello '.$user['displayName'], 'text/plain');
}

Start with the current request flow described in the issue, especially RestResponse methods and the endpoint resource('/users/me')->get()->value() call. Compare it with the proposed generator-based await() handler and determine how deferred reading should interact with the response methods. Done means asynchronous invocation can avoid blocking a single-threaded xp-forge/web server while preserving the existing response behavior.